Improve exec command processing

- Make exec print out an error if execve() failed for some reason
- Pass init's environment, created by the export keywords, to the exec'd
  command, which is helpful when running shell scripts (for PATH) and/or
  am (for BOOTCLASSPATH) from an exec statement.

Conflicts:
	init/builtins.c
Merge remote-tracking branch 'cyanogen/cm-10.1' into cm-10.1
Merge branch 'cm-10.1.1' into martincz
Merge remote-tracking branch 'mm/cm-10.1' into HEAD